1. Field of the Invention
The present invention relates to a fixing apparatus and an image forming apparatus, and more particularly to a fixing apparatus for fixing a toner image onto a recording sheet and an image forming apparatus including the fixing apparatus.
2. Description of the Related Art
An image forming apparatus by means of an electrophotographic method or the like is provided with a fixing apparatus for heating an unfixed toner image carried on a recording sheet so that the unfixed toner image is fixed onto the recording sheet. The fixing apparatus is composed of various rollers such as a fixing roller for heating the recording sheet, a pressurizing roller that is driven by the fixing roller to be rotated and has a nip portion formed between the fixing roller and itself, and further an external heating roller for heating the pressurizing roller.
Then, as a method for cleaning paper dust and the like attached to the surface of, for example, the fixing roller that constitutes the above-described fixing apparatus, a method for pressing a cleaning pad on the surface of the fixing roller is known. For example, Japanese Laid-Open Patent Publication No. 62-123486 describes a technique for giving a stronger pressing force of the cleaning pad to a downstream side than to an upstream side with respect to a rotational (radial) direction of the fixing roller in order to keep a pressure of the cleaning pad to the fixing roller constant, in a fixing apparatus.
Some of such cleaning pads are formed by attaching a heat-resistant felt, with a double-stick tape, to a support member such as a leaf spring having a square-U-shaped cross-section made of stainless steel, phosphor bronze or the like. The heat-resistant felt is pressed on the surface of the fixing roller with elastic force of the leaf spring to clean the surface of the fixing roller.
FIG. 10A to FIG. 10C are diagrams showing appearance of a support member that constitutes a conventional cleaning pad. FIG. 10A is a top view of the support member, FIG. 10B is a rear view of the support member, and FIG. 10C is a bottom view of the support member. In the diagrams, 100 denotes the support member. The support member 100 is pressed against a pressure-contacted roller such as a fixing roller or an external heating roller via heat-resistant felt, and used under a high-temperature environment, thus a strain due to thermal expansion occurs. Consequently, a plurality of holes or slits 100a (hereinafter, referred to as holes 100a) are formed equally at even intervals at a bending portion of the leaf spring folded back so as to have a square-U-shaped cross-section along a long side (thrust) direction of the pressure-contacted roller, and the plurality of holes 100a make it possible to suppress thermal expansion.
FIG. 11 is a diagram showing a distribution state of a surface temperature of the pressure-contacted roller against which the cleaning pad is pressed. In the diagram, a longitudinal axis shows the surface temperature in a thrust direction of the pressure-contacted roller, and a lateral axis shows a position in the thrust direction of the pressure-contacted roller. In this manner, since the surface temperature of the pressure-contacted roller is higher at a center part compared to both edge parts, the cleaning pad pressed against the pressure-contact roller tends to have a greater thermal expansion and easily have a larger strain at the center part compared to the both edge parts. And, since the contact pressure of the cleaning pad against the pressure-contacted roller is changed in the thrust direction due to the strain, problems such as cleaning defection, unevenness of cleaning have occurred, and caused a problem to contaminate a recording sheet.
On the other hand, the technique described in the above Japanese Laid-Open Patent Publication No. 62-123486 aims to stabilize the contact pressure of a cleaning pad with respect to a radial direction of a fixing roller, however, considerations are not made for stabilizing the contact pressure with respect to a thrust direction of the fixing roller. Namely, it is impossible to suppress thermal expansion with respect to the thrust direction of the fixing roller.
Additionally, it is considered that a suppression effect on a thermal expansion is approximately the same at a center part in which the thermal expansion is great and at both edge parts in which the thermal expansion is small, when a plurality of the holes 100a are equally formed at even intervals in a long side direction of the bending portion like the support member 100 shown in FIG. 10A to FIG. 10C. Consequently, even though it is possible to suppress the thermal expansion at the both edge parts in which the thermal expansion is small by means of the holes 100a, it may be impossible to sufficiently suppress the thermal expansion at the center part in which the thermal expansion is great. Thus, a residual strain due to the thermal expansion may occur at the center part of the support member 100.